<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

        #d1{
            color: red;
        }
        div{
            /*!important作用是提升优先级*/
            color: blue !important;
        }

    </style>
</head>
<body>
<div id="d1">
    <p>这是个p标签</p>
    <span>这是div里面的span</span>
    <a href="">超链接</a>
</div>
<span>这是div外面的span</span>

</body>
</html>